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STUDY OF COSMIC RAY INTENSITY MEASURED BY MARINER I1 AND BY GROUND NEUTRON MONITORS DURING THE FORBUSH DECREASE OF SEPTEMBER 30, 1962

U. R . Rao

Southwest Center f o r Advanced Studies

Dallas, Texas 75230 P. 0. Box 30365

A l e t t e r

To be submitted t o

JOURNAL OF GEOPHYSICAL RESEARCH

ACKNOWLEDGEMENT - This research has been supported by the National Aeronautics and Space Administration undaer grant NsG-269-62.

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1

STUDY OF COSMIC R A Y INTENSITY MEASURED BY MARINER I1 AND BY GROUND NEUTRON r

MONITORS DURING THE FORBUSH DECREASE OF SEPTEMBER 30, 1962

bY U. R. Rao

Southwest Center f o r Advanced Studies P. 0. Box 30365, Dal las , Texas

The p l aae t a ry probe Nariner I1 which covered a r a d i a l d i s t a n c e of about 0.25 AU

32 i t s journey t o the o r b i t of Venus contained ins t ruments t o measure cosmic r a y

i n t e n s i t y a s w e l l a s a plasma probe to measure t h e f l u x and v e l o c i t y of s o l a r wind.

T- A L , ~ cosziic r a y i t e n s i t y measurements were c a r r i e d ou t by an a rgon- f i l l ed i n t e g r a t -

i a g ior. ization chamber, a halogen quenched, g l a s s Geiger-Mueller counter tube with

a macched s t a i n l e s s steel s h i e l d and a second G.M. counter with a beryl l ium sh ie ld .

All t h e t h r e e cosmic r a y instruments had approximately t h e same threshold energy

of 10 mev for protons. The desc r ip t ion of t h e experiment and some of t h e r e s u l t s

of both t h e cosinic r a y measurements (Neher and Anderson, 1964; Anderson,, 1964) and

t h e plasma probe experiment (Snyder, Neugebauer, and Rao, 1964) have been publ ished

elsewhere.

The probe y ie lded u s e f u l d a t a almost cont inuously from August 29 through t h e

end of December, 1962. During t h i s period, t h e r e was on ly one d i s t i n c t Forbush

decrease, which occurred on 30 September, 1962 (Day 273), when Mariner I1 w a s ap-

proximately 0.1 AU from t h e e a r t h and o r i en ted almost (within 50) along t h e ea r th -

sun l i n e . I n t h i s no te we examine the d a t a from va r ious instruments on Mariner I1

and t h e d a t a from ground neut ron monitors during t h e above-mentioned Forbush decrease.

F igure 1 shows t h e time s e r i e s of t he hourly mean cosmic r a y i n t e n s i t i e s

aeasured by Mariner I1 i o n chamber; the hourly mean neut ron i n t e n s i t i e s a t Deep

River, E t . Wellington, and Mawson; the hourly mean s o l a r wind v e l o c i t y , and t h e

t h r e e hour ly Kp i n d i c e s f o r t h e per iod 29 September (Day 272) thmugh 5 October,

1962 (Day 278). The onse t of t h e Forbush decrease occurs a t 0830 U,T.as seen by

t h e <on chamber i n t e n s i t y on Mariner 11.

l a r g e s t a t i s t i c a l e r r o r s i n t h e neutron monitor i n t e n s i t i e s , it i s n o t - p o s s i b l e

t o i d e n t i f y c l e a r l y t h e a n i s o t r o p i e s and t h e onse t t i m e d i f f e r e n c e s a t t h e ground.

However, t h e c o r r e l a t i o n between t h e changes i n neut ron i n t e n s i t y a t ground s t a t i o n s

Due t o t h e small decreases (-3.0%) and

I - 2-

S L C ~ I as Jsep R i v e r and the changes i n ion chamber i n t e n s i t y on Mariner I1 i s very

il..g11 (0.91 i 0.02). The r a t i o of Forbush decrease observed by t h e ior, chamber t o

:ha: o j s e n e d by he neucron monitor, obtained from zhe r e g r e s s i o n l i n e i s 3.88 2 0.01.

Sixz i\,aricer I1 i s almosc along t h e earth-sun l i n e acd i s less than 0 .1 AU from the

esrri;, ,h is Yatio i s t r u l y r ep resen ta t ive of t h e r a t i o of primary i n t e n s i t y changes

, - .

LIl.3s~he:e ‘io t he changes of neutron i n t e n s i r y a t Deep River. - - tZe zk. eazzh’s =I+- - -

Lack af o 5 s e w a t i o n of any s p a t i a l gradient of i n t e n s i t y (Anderson, 1964) during

25.2 days 239 through 292 lends stroiig suyport t o t h e abovz s t azeaen t .

F r m bsl loon obsezvations durin, t h e same period (1959-1963), Nerurkar and

Iie3ber (1’364) have S~OWLI rilat che slope of t h e long term and s h o r t term v a r i a t i o n s

o b s e i i ~ ~ d a; bal loon altitudes mer Kir;nez?olis t o t h e corresponding neutron inten-

s i t y v a r t z z i o n s tit Deep River is very much de?er,dent on t h e level of observed inten-

s l z y ; t h e s;o?e being l a r g e r when t h e level of i n t e n s i t y a t D e e ? River i s higher ,

i n i i c s t i n g :he presence of a d d i t i o n a l low eae-rgy r a d i a t i o n .

i n t e n s i t y ar 3eep River p r e v a i l i n g p r i o r t o t h e Forbush dec res se of 30 Septenber,

i962, a s lope of 3.75 i s derived f o r the long term v a r i a t i o n , u s ing t h e Nerurkar and

W25Ser r e s u l r s .

chamber i n t e n s i t y on YarLner I1 and Deep River neutron i n t e n s i t y during t h e For3ush

&crease 0, 30 September, 1962, we conclude Ehat t h e speccrum of v a r i a t i o n during

For t h e level of neutron

i CorrLparir,g t h r s with the observed s lope of 3.88 - .01 between ion

t h e A o r t tern f l u c t u a t i o n s , such as Forbush decreases , i s sirnila-r t o t h e specZrm

of v a r i a t i o n during long te;n changes. Anderson (1964), on t h e o t h e r hand, f i n d s

t h a t t h e r a t i o of v a r i z t i o n of t he d a i l y average Mariner i o n chamber rates t o varia-

t i o n s of t he dai;y average Deep River r a t e f a l l s between 2 and 3 d3rir.g d i f f e r e n t

per iods of observat ion. It must, however, be rexembered chat af’ier day 292, Xariner ii

r a p l d l y d r i f t e d away from t h e earch-sun l i n e , and away from t h e e a r t h toward Vems. .A

systematic i n t e n s i t y g rad ien t o f Lpproximately 9”/,A’J has been a s t ima tec by Anderson

(1964) from a ccnparison of ion chamber i n t e n s i t y with Deep River nclutron i n t e n s i c y .

Since t h e s e f a c t o r s w i l l c l e a r l y a f f e c t t he observed r a t i o s , t h e r a t i o oStained by

- 3- *

Anderson, even though less than t h a t observed dur ing t h e Forbush decrease o r t h a t

obtained by Nerurkar and Webbzr, is not i n c o n t r a d i c t i o n with these r e s u l t s .

The percent change i n i n t e n s i t y of a secondary component of type i, recorded

a t l a t i t u d e A and he ight h can be expressed as 00

- wi(h,E) dE J E X -'-' 100 6Nf(h)

=

b D E 100 L

where E i i s the v e r t i c a l cu to f f energy a t l a t i t u d e A , wft(h,E) i s t h e coupl ing

cons tan t f o r secondary component. i defined as

d i f f e r e n t i a l energy spectrum of primaries and mi(E,h) is the"mu1t ip l ic i ty function. ' '

D@) mf(E,h) where D(E) is t h e N p )

S imi l a r ly , we can express t h e percent change i n i n t e n s i t y of primary r a d i a t i o n such

as t h e one measured by Mariner I1 i o n chamber as 00

100 6 N I(E) *E] D(E)dE P= (2 1

NP I(E) D(E)dE

where I ( E ) i s t h e i o n i z a t i o n rate pe r u n i t omnid i rec t iona l f l u x of p a r t i c l e s wi th

energy E. [I(E) equals u n i t y f o r Geiger counters . e%in is t h e threshold of t h e

Mariner I1 inst ruments (10 Mev f o r protons). 1

Assuming t h e spectrum of v a r i a t i o n

t o be of t h e form AE P and using t h e coupling c o e f f i c i e n t s f o r sea level neutrons

a t 50° l a t i t u d e given by Webber and Quenby (1959) and t h e r ecen t IMP-1 r e s u l t s on

primary spectrum (McDonald and Ludwig, 1964), we have ca l cu la t ed t h e r a t i o of per-

c e n t change of cosmic r a y i n t e n s i t i e s measured by Mariner I1 i o n chamber and Geiger

counters t o t h a t observed by Deep River neut ron monitor f o r d i f f e r e n t exponents of

t h e spectrum of v a r i a t i o n . Figure 2 shows t h e t h e o r e t i c a l l y c a l c u l a t e d r a t i o of

t h e pe rcen t change observed by Mariner I1 i o n chamber and t h e Geiger counters t o

t h e pe rcen t change of cosmic r a y i n t e n s i t y measured by Deep River neut ron m o n i t o r

f o r v a r i o u s va lues of t h e exponent 8.

. - 4-

I n Figure 3 i s p l o t t e d t h e time s e r i e s of t h e 6 hour ly mean cosmic r ay in t en -

s i t i e s neasilred by Hariner I1 i on chamber, Be , and s teel sh i e lded G-M counters

aboard ?iariner T I and t h e Deep River neutron i n t e n s i t y f o r t h e period 29 September

(272) through 13 October 1962 (286),

as t h e changes of i n t e n s i t y i n va r ious instruments, c o r r e l a t e ve ry w e l l a s seen

The onse t of t h e Forbush decrease, as w e l l

from Figilre 3, t h e c o r r e l a t i o n c o e f f i c i e n t between any two i n t e n s i t i e s being a t

leas: 0.85 ? .05. However, t h e s lopes of t h e i n t e n s i t y changes observed by t h e

two G-X counters , both having similhr energy respozse t o t h e neutron i n t e n s i t y +

change a t Deep River, do n o t ag ree very w e l l , t h e s lopes being 2.39 - .02 and

3.05 $. .02. S i m i l a r d i sc repanc ie s have been noted by Anderson (1964). Hence, we

have cunsicked t h e mean of t h e two values, namely 2.72 2 .02, as t h e proper r a t i o

of t h e changes of G-M counter i n t e n s i t y a t Mariner I1 to t h e changes of neutron

i n t e n s i t y a t Deep River.

Using r h e two observed ratios, one between t h e ion chamber and Deep River

+ neutron i n t e n s i t i e s (3.88 - .01) and the o t h e r between G M counter and Deep River

+ neutron i n t e n s i t i e s (2.72 - . O l ) , we obtain from Figure 2,two estimates f o r 0,

namely -0.41 ? 0.01 and -0.48

spectrum o f v a r i a t i o n during t h e Forbush decrease w a s of t h e fo-m AE

.01 r e spec t ive ly . W e conclude t h a t t h e energy

-0.45 -t .or,

Due t o l a c k of good agreement between the two Geiger counters , we a r e , however,

unable t o determine whether t h e i n t e n s i t y changes during t h i s Forbudh decrease was

energy o r r i g i d i t y dependent.

1~1 Figure 1 i s a l s o p l o t t e d t h e time series of t h e hour ly mean s o l a r win6

v e l o c i t y (reversed scale), measured by t h e plasma probe i n Mariner 11, and t h e

t h r e e hour ly % i nd ices .

plasma bulk v e l o c i t y , by t h e v a l u e f o r the channel with t h e l a r g e s t measured c m -

r e n t . The e r r o r a r i s i n g from t h i s appyoximation (Snyder, Neugebauer, and Rao) i s

n e g l i g i b l e .

wind v e l o c i t y follows the t i m e series of i o n chan5er cosmic r a y i n t e n s i t y ve ry

c l o s e l y .

For t h e ana lys i s presented he re we have approxiinated t h e

From t h e f i g u r e it i s c l e a r t h a t t h e i n v e r s e time series of the s o l a r

The inverse --relation between t h e changes i n plasma v e l o c i t y and t h e

- 5-

char,ges iil ion chamber cosmic ray intensity is very high, being -0.98 2 .01. A

cmqarlson of the plasma velocity chariges with the changes of ion intensity reveals

char each decrease in ion chamber intensity is correlated with a corresponding

Lacrease in plasma velocity, as if each individual burst of plasma cloud interacts

i S 7 i t f ; the interplanetarv medium to produce a corresponding decrease of cosmic ray

2- iALensitv. i

tensity is very high.

Similarly, .the correlztia between Kp indices and the ion chamber in-

Even though the correlation of ion chamber intensity with

plasma velocity, or 5, is very high during the Forbush decrease, the correlation is practicaliy nonexistent on a long term basis, (Snyder, Neugebauer, and Rao).

Assming that the Forbush decrease has made a complete recovery when the intensity

reaches wiihin 0.5 percent of the pre-evznt value, it can be seen from Figure 1

that even though both $ and plasma velocity -recove, to their normal value in about

two days (i.e, on day 276), the cosmic ray intensities continue to remain depressed.

The xeutron intensities at various statlons recover on day 277. The ion chamber

ixtensity and the G M counter intensities on Yfriner 11, however, recover only on

day 285 or 286 (Figure 3), showing that the recovery is energy dependent, the low

energy cosmic rays recorded by the Mariner I1 ion chamber and G-M counters recover-

ing last.

In summary, we draw the following important conclusions:

(1) The changes in ion chamber intensity measured in space during the Forbush

decrease of 30 September 1962 are very well correlated with the changes of neutron

intensity at Deep River and at other neutron monitor stations. The neutron inten-

sity at the ground, however, recovers much faster than the ion chamber intensft.y,

indicating that the intensity of low energy cosmic rays has still aot recovered

to its pre-decrease value.

(2) Both the short term variations, such as Forbush decreases, and the long

term variations have a similar spectrurn of variation.

(3) The spectrum of variation during the Forbush decrease may be represented

-6-

The changes i n cosmic ray i n t e n s i t y a r e very w e l l c o r r e l a t e d withf l the changes

i n s o l a r wind v e l o c i t y during the Forbush decrease. The e f f e c t i s as though each

ind iv idua l b u r s t of plasma cloud interacts with t h e i n t e r p l a n e t a r y medium t o produce

a corresponding decrease i n the cosmic r ay i n t e n s i t y .

AC KNOWLED GMlZ NTS

I am gra t e fu l to D r s . H. Anderson, C . W. Snyder, and M. Neugebauer of the

Jet Propulsion Laboratory, Pas,adena, California, f o r permission t o use the

Mariner I1 data. I am a l so indebted to D r . K. G. McCracken f o r he lpfu l suggestions.

The work reported here .was supported by the NASA Research Grant NSG269-62.

REFERENCES

Anderson, H. R . ; "Ionizing Radiation Measured Between Earth and Venus by Mariner 11," paper presented a t the In te rna t iona l Space Science Sympo;ium (COSPAR), Florence, 1964. (In Press)

Fenton, A. G., K. G. McCracken, D . C. Rose, and B. G. Wilson; "The Onset Times of Forbush-type Cosmic Ray I n t e n s i t y Decreases," Can. J. Phys; 3 7 , 970-982, 1959.

NcCracken, K. G.; "The Cosmic Ray Flare Effect," J. Geophys. Research, 6 7 , 447- 458, 1962.

McDonald, F. B.,.and G. H. Ludwig; "Measurement of Low Energy Primary Cosmic Ray Protons on IMP-1 S a t e l l i t e , " 1964. (In Press)

Neher, H. V., and H. R. Anderson; "Cosmic Ray I n t e n s i t y a t Thule, Greenland, during 1962 and 1963 and a Comparison with Data from Mariner 11," J. Geophys. Research, 6 9 , 807-814, 1964.

Nerurkar, N. , and W. R. Webber; "Observations of Primary Cosmic Ray Variations Using Ion Chambers and Geiger Counters," J. Geophys. Research, 69 , 815-830, 1964.

Snyder, C . W. , M. Neugebauer, and U. R. Rao; "The Solar Wind Velocity and I ts Correlat ion with Cosmic Ray Variations and with Solar and Geomagnetic Activity," J. Geophys. Research, 68 , 6361-6370, 1963.

CAPTIONS FOR FIGURES

Figure 1 Time s e r i e s showing t h e ( a ) hourly mean cosmic r a y i n t e n s i t y recorded by Mariner I1 ion chamber (b) hourly mean s o l a r wind v e l o c i t y , (c) hourly mean neutron i n t e n s i t i e s recorded a t Deep River, M t . Wellington and Mawson, and (d) t h e 3 hourly Kp ind ices f o r t he per iod September 29 (272) through October 5, 1962 (Day 278).

Figure 2 Theore t i ca l curve showing the r a t i o of percent change cosmic ray i n t e n s i t y observed by Mariner I1 ion chamber and geiger counters t o the pe rcen t change neutron i n t e n s i t y a t Deep River as a func t ion of 0, f o r a spectrum of the type AE8.

Figure 3 T i m e s e r i e s of hourly mean cosmic ray i n t e n s i t i e s measured by (a) Mariner I1 ion chamber, (b) beryll ium and s t a i n l e s s s t e e l shielded ge ige r coun te r s aboard Mariner 11, and (c) t h e neutron i n t e n s i t y a t Deep River f o r t h e period September 29 (272) through October 13, 1962 (286).

' 1 ' I l l 1 1 ' I l l 276 277 2 78 272 273 274 2 75 ' 1 1 1 1 ' s I

. . . . . . . . . . . . . . , . . . . . . .

.... 0

MARINER E ION CHAMBER DATA - 2.0

-4.0

-6.0

- 8.0

' 10.0

* I-

0 I2 0 12 0 12 0 12 0 I2 0 I2 0 12 U.T. 272 273 214 275 276 277 278

DAY NUMBER

1400 0 I c D

450 4 I m D

500 2

0 I- D

D <

600 E 1

550 E

4 < x 3 \

650

700 n -

750

. . .

10.0 I

THEORETICAL CURVE A I GEIGER COUNTER A I DEEP RIVER

9.0 - ,*

4 4

8.0 - L

b f ION CHAMBER

7.0 - A I DEEP RIVER

6.0 -

5.0 - Y

4

4.0 - OBSERVED RATIO 3.

3.0 - OBSERVED RATIO 2.7

2.0 -

1.0 - /

+/

0.0 I +0.4 +0.2 0.0 -0.2 -0.4 -0.6 -0.8 -I .o

EXPONENT

v, w t v, z W I- Z -

4 [r

: v) 0 0

LL 0

W (3 Z I 0

I- z W 0 lK w Q

a

. ,

b

t 2.0

0.0

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2

# -2.0

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0

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-8.0 4 z

0.0

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s -2.0

8 -4-0 12

fi -6.0

w

w W

0 J

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0.0

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"6-6.0

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-8.0 I 0.0 "

2

I-

2

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z $ -1.0

a 2 -2.0 a PW # -3.0

272 213 214 215 276 277 270 279 280 281 282 203 284 285 286

DAY NUMBER